Noncontact printers which utilize charged droplets are generally known in the art as shown by U.S. Pat. Nos. 3,373,437 to Sweet et al; 3,560,988 to Crick; 3,579,721 to Kaltenbach; and 3,596,275 to Sweet. Typically, fluid filaments of ink, dye or the like pass through the orifices of an orifice plate having an array of individually-controllable electrostatic charging electrodes disposed downstream of the orifice plate along the "droplet formation zone." In accordance with known principles of electrostatic induction, each fluid filament assumes an electrical charge opposite in polarity but related in magnitude to the electrical charge of its respective charging electrode. When a droplet of fluid separates from the filament, an induced electrostatic charge, a scalar quantity is trapped in the droplet. Subsequently, the charged droplet passes through an electrostatic field, a vector quantity. The electric field is oriented so that the droplet is deflected from the normal path towards the droplet catching structure. Uncharged droplets proceed along a normal path and are deposited upon a receiving substrate. Typical prior art ink jet printing systems use a random droplet system whereby droplets in a linear array break off naturally in accordance with Rayleigh distribution formulae or as a result of randomly-applied energetic stimulation.
A problem exists with droplet catching structures used in prior art ink jet systems in that such structures have not been as effective as desired in applications requiring the capture of closely spaced, high-density droplet streams. It is desirable that the catcher face be designed such that the individual droplet streams will "wet out" the surface of the catcher structure as they impact on the face to form a uniform, adhering film of liquid which will follow the profile of the drop catcher face. If the jets are closely spaced, i.e., in a "high-density" configuration, it becomes virtually impossible to form the desired uniform layer of liquid on the catcher structure because the droplets tend to "spill off" the face, thereby destroying the regularity and clarity of images formed on the receiving substrate. The problem is particularly acute in systems using a catcher face structure which is substantially vertical or inclined downwardly to only a small degree in the direction toward the normal path of droplet flow.
Similarly, for high-density orifice applications in which the catcher face is inclined to a greater extent, i.e., more oblique to the path of droplet flow, the momentum of the individual droplets becomes too great and the droplet streams tend to create a "splash back" or "misting" condition. Such misting occurs at or near the point of contact with the catcher face and tends to collect upon the electrodes, the print, and other areas of the fluid jet device to the detriment of the overall printing operation.
An additional problem exists with prior art drop catcher structures in that their effectiveness depends to a significant degree on the ink jet pressure exerted on droplet streams issuing from the orifice plate. That is, a configuration which is acceptable for a particular ink jet pressure may not be satisfactory at a higher pressure due to the increase in momentum of droplets issuing from the orifice plate. In addition, at elevated pressures, the possibility that splashback will occur is significantly greater.
The present invention substantially alleviates the above problems by providing a drop catcher structure for capturing deflected, closely adjacent, i.e., high-density, droplet streams issuing from an orifice plate with little or no loss of clarity or uniformity of images formed on the receiving substrate.
It has now been found that high density droplet streams issuing from an orifice plate can be effectively caught using a catcher face configuration having at least three separate but interrelated surfaces. The various embodiments contemplated by the present invention each share a common structural feature--a planar drop-catching surface which is inclined downwardly in a direction towards the vertical path of droplet streams issuing from the orifice plate. It has been found that the amount of inclination for high-density droplet streams should range between 8.degree. and 70.degree. relative to the normal path of droplet flow, depending on the fluid pressure. By using a drop catcher face having such downward inclination--that is, a planar surface sloped toward the droplet path--the high-density droplet streams deflected onto the surface will effectively "wet out" the surface of the catcher face to form a uniform flowing layer of liquid which will follow the profile of the drop catcher face into the ingesting blade and a suitable vacuum slot.
In exemplary embodiments using a catcher face structure in accordance with the present invention, the droplets issue from a linear array of orifices having a spacing in the range of between 5 mils and a diameter of about 1.3 mils to a spacing of approximately 14 mils and a diameter of about 4 mils, and are guided electrostatically by planar electrode means which provide a transverse deflection field through which the droplets pass. An individual droplet is thus either deflected by the transverse deflection field and caught on the catcher face or permitted to continue on to strike the receiving substrate.
It has now been found that for certain embodiments of the present invention. particularly those used in printing applications for rugs, carpets and the like, individual ink droplets which strike a catcher face having a downward and outward inclination of less than 12.degree. (for an ink jet pressure of approximately 15 pounds per square inch), tend to "skip off" the fluid film that forms on the catcher face and thus not be caught by the catcher. At angles greater than 12.degree. but less than 24.degree. (and an ink jet pressure of approximately 15 PSI), the individual jets are cleanly caught and a film of liquid forms around the front lip portion of the catcher structure.
In other embodiments of the present invention. particularly those suitable for solid shade applications on lighter weight textile fabrics, it has been found that if the individual droplets strike a catcher facing having a downward and outward inclination of between 26.degree. and 70.degree. (preferably 30.degree.) for ink jet pressures in the range of 2 to 5 PSI, the individual jets will not splash or "skip off" the catcher face and will be cleanly caught by the catcher structure. Within that preferred range of surface inclinations, the resulting fluid film remains uniform and stable and forms a smooth flowing layer of liquid which follows the profile of the catcher face into the vacuum slot.
The momentum of individual droplets in the direction parallel to the catcher face must not, however, exceed certain levels depending upon the total kinetic energy of the stream, the droplet size and surface tension of the droplets. Thus, if the pressure of the ink jets is increased (thereby increasing the total momentum of the droplets), the angular position of the catcher structure will change. At higher ink jet pressures, catcher structures in accordance with the invention must use lower angles of inclination. For example, at a 30 PSI ink jet pressure, the preferred angle of inclination relative to the path of droplet flow will be approximately 8.degree. with a maximum possible angle of inclination of only 16.degree.. In contrast, for low pressure applications in the range of 2-5 PSI, the angle of inclination may be as high as 70.degree. due to the reduced velocity of the ink droplets.
Thus, it is an object of the present invention to provide for an improved catcher structure for ink jet systems using closely spaced, i.e., high-density, jets such as those used in random droplet systems.
It is a further object of the present invention to provide an ink jet catcher system in which high-density deflected droplets are caught by the catcher face to form an adhering film of fluid flowing down the front face of the catcher structure.
It is still a further object of the present invention to provide a catcher structure which will prevent "splashback" or "misting" of deflected droplets which would otherwise destroy the reliability of the printing operation and affect the clarity of the print on the receiving substrate.
It is still a further object of the present invention to provide a catcher structure which will operate effectively over a wider range of ink jet pressures.